It is known to use an optical fiber to sense an external disturbance or other effect external to the fiber. For example, U.S. Pat. No. 6,459,486 is directed to a sensing system in which an interferometer system is used to detect a time varying disturbance. In such a system, the disturbance causes a phase shift in sensing signals carried by the waveguide, which phase shift can then be detected interferometrically. However, if the event creates only a weak disturbance to the fiber, the resulting phase shift is also likely to be weak, with the result that the disturbance may be difficult to detect.